1. Field of the Invention
This invention is related generally to phase-locked loops and specifically to phase-locked loops having components for assuring initial lock to an incoming signal.
2. Description of the Prior Art
There are many applications where it is desirable to synchronize, or lock, the phase and frequency of a locally produced signal to the phase and frequency of a received signal. For example, such synchronization is required for the demodulation of single sideband signals. The required synchronization may be obtained by using a phase-locked loop, hereinafter PLL.
A PLL is typically comprised of a phase detector, a low-pass filter, an amplifier, and a voltage controlled oscillator, hereinafter VCO. The phase detector compares the phase of an input signal to the phase of a variable frequency output signal which is produced by the VCO. The phase detector produces an output voltage that is representative of the instantaneous difference between the input signal and the output signal. The output voltage of the phase detector is filtered, amplified and input to the VCO. The frequency of the output signal produced by the VCO varies with the voltage input thereto, which voltage is representative of the phase difference between the input signal and the output signal. In this manner the phase and frequency of the output signal is locked to the phase and frequency of the input signal.
In designing a PLL the bandwidth chosen must be a compromise between competing design criteria. It is desirable to have a wide bandwidth thus allowing for initial synchronization, or capture, over a wide range of frequencies. However, a wide bandwidth will have poor noise rejection characteristics and thus the output signal may not precisely track the input signal. In order to increase the precision with which the output signal tracks the input signal it is necessary to decrease the bandwidth. This decrease in bandwidth will, however, reduce the capture range of the PLL. Thus, the bandwidth chosen for the typical PLL must be a compromise between a wide bandwidth allowing for fast acquisition over a wide range of frequencies and a narrow bandwidth allowing for noise-free tracking of the input signal.
One prior art technique for avoiding this compromise is to have two loops responsive to the received signal. One loop has a large bandwidth for fast acquisition over a wide range of frequencies; the second loop has a narrow bandwidth for error-free tracking. A switch, responsive to the difference between the input signal and the output signal, determines which loop is connected to the VCO. When the difference is large, the loop having the large bandwidth is connected to the VCO. Conversely, when the difference is small, the loop having the narrow bandwidth is connected to the VCO.